Battery cells are not always used at room temperature. For example, battery cells used in communication satellites are after exposed to temperatures ranging from about 20.degree. C. to 0.degree. C. The internal temperature of a battery cell can have a pronounced effect on its performance. Thus, many battery cells which are exposed to temperature extremes in use must be tested at those temperatures in order to predict accurately whether they will perform adequately.
The temperature of some battery cells can also change markedly during electrical cycles of charging and discharging.
Nickel-hydrogen (pressure vessel) battery cells, which are used in satellites, behave in this manner. When the temperature of such battery cells exceeds about 20.degree. C., their efficiency begins to drop rapidly. Apparatus used to test such battery cells at simulated use temperatures must also be able to eliminate the effects of heat generated by the battery cell during testing, in order to ensure the accuracy of the performance predictions.
A battery cell can be distinguished from a battery in that a battery cell has an inherent voltage which is independent of cell size and is determined only by the chemical nature of its components, less resistive current losses. A battery on the other hand is always composed of two or more battery cells. The life of a group of interconnected battery cells can depend, among other things, on uniformity of the capacity of each. To ensure that battery cells in such installations are evenly matched, the performance of each battery cell needs to be accurately known in advance. The accuracy of such performance predictions can be critical in some applications. For example, the battery cells used in a communication satellite are expected to have a life while in orbit of 15 years or more. The accuracy of predicting the functional life of a satellite can be critical, one reason being the great expense incurred in placing the satellite into space. Therefore, battery cells like these are typically 100% tested (i.e. each battery cell is tested) at the temperature desired, rather than lot or sample tested. To reduce the costs of such testing, the apparatus used to test battery cells 100%, or in relatively large quantities, should be structured to enable the battery cells to be quickly mounted, tested and removed.
A number of devices have been developed to heat or cool battery cells during use or testing. One device employs radiative and conductive cooling by means of a close-fitting, chilled metallic test block surrounding the battery cell. However, it is difficult and time consuming to establish uniform conductive contact for a large group of battery cells with such a close-fitting test block. Such metal test blocks are commonly used in the aerospace battery cell industry to test battery cells like nickel-hydrogen battery cells which are typically housed in a cylindrical pressure vessel having hemispherical ends. When such a battery cell is tested in a metal test block, heat transfer and the speed of testing is impeded even more by a dielectric wrap which is used to prevent shorting of the pressure vessel, through which the heat must pass. In addition, chilled air in the test fixture naturally falls in a gravity environment to the base of the battery cell, making the lower end considerably colder than the upper end. This is an undesirable condition.
Another common method for testing a battery cell at a particular temperature involves cooling the battery cell through a thermal sleeve which is attached to the battery cell with a thin layer of insulating material such as a rubber. The sleeve/battery cell assembly is then affixed to a chilled metallic base plate. However, this apparatus, which requires a chilled base plate made to fit the sleeve, is expensive to make and slow to use. In addition, it is often impractical to mass test battery cells with such apparatus because the test sleeves are difficult to remove.